Peffley: Use wood ash in gardens with limitations

Fireplaces were busy last week warding off frigid outdoor temperatures. After logs are burned and fires die, only ashes remain. Is there a use for the ashes in the garden?

Wood ash can be used in the garden or landscape but with limitations. There are positives of using wood ash as an amendment, but concomitant with its use are negatives.

Ash that remains after wood is burned will contain minerals leached from the soil by the tree. As trees grow, nutrients in the form of mineral elements are taken up by the roots and translocated throughout the tissues. Minerals in the soil will have been moved into the wood and stay in the logs when harvested.

Wood ash contains most of the mineral elements required for optimal plant growth and development. But it is important to recognize which elements remain, in what quantity, and the chemical composition of the living trees from which the wood was cut.

Chemical composition

When wood is burned the elements nitrogen and sulfur are volatized as gases, lost to the atmosphere and not available for plant use. Minerals remaining in the highest quantities are calcium, potassium and magnesium with trace amounts of iron, zinc, copper, nickel, selenium and other minerals. The two minerals in highest quantity are calcium and potassium.

Calcium is essential for root development and strengthening cell walls. Calcium in wood ash is in the form of calcium carbonate, better known in our region as caliche. When caliche hardens it becomes impervious to water — think when something becomes calcified (as in bones). Calcium carbonate is also used as a liming agent to raise the pH of acidic soils. Since our soils are alkaline with sufficient calcium for optimal plant growth, adding an agent that raises the pH even higher would be detrimental or harmful to plant growth.

Potassium is used by plants in photosynthesis and movement of substances in tissues. Generally potassium levels of our soils are fairly high, as potassium is not readily leached from soils in our semi-arid region. Adding surplus potassium may be detrimental or harmful to growth.

Type of wood:

There are two types of wood: hardwood and softwood. The types of wood do not refer to density, rather hardwood comes from flowering trees, such as oaks or pecan, while softwood comes from evergreen conifers, such as pine or fir. Hardwoods produce three times as much ash per cord as do softwoods, generally containing more potassium and less calcium.

Uses of wood ash:

Anecdotally, wood ash is used to deter insects such as borers from invading fruit trees and slugs and snails from feeding on succulent plant tissue. Ash sprinkled around the base of plants may discourage pests such as squash bugs that crawl on the ground before laying eggs.

Keep in mind that ash increases soil pH and becomes rigid once wet, reducing any insecticidal properties. Accordingly, use wood ash sparingly in the garden.

Some information from forestry.gov.uk

ELLEN PEFFLEY taught horticulture at the college level for 28 years, 25 of those at Texas Tech, during which time she developed two onion varieties. She is now the sole proprietor of From the Garden, a market garden farmette. You can email her at gardens@suddenlink.net.

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